The most common positioning system used in prior art expensive drives, is the moving coil system in which a multi-turn electromagnetic coil is mounted on the carriage and positioned in a flux gap of a fixed, permanent magnet structure. By controlling the polarity and amplitude of current flowing through the coil, a thrust of controlled magnitude and direction is generated. A second element of this system is a position encoder attached to the carriage which is usually optical, but occasionally inductive. Electronically controlled forces generated by the electromagnetic coil are used to guide the carriage to the positions indicated by the position encoder. Most of these systems also require a velocity transducer to sense the velocity of the carriage. Because of the many elements of this closed loop system, it is quite expensive and therefore it is not economically competitive for very low cost positioners such as those used in the typical floppy disc drive.
In low cost drives, open loop systems are preferred, in which a positioner moves directly to inherent, predetermined discrete positions with no electronic verification that the desired position has been reached. Such systems cost little and work well provided their torque and inertia limits have not been exceeded.
The simplest of these open loop systems is a linear stepper. One version in commercial production utilizes a long cylindrical armature of soft iron attached to the carriage upon which the transducers are mounted. Into this armature are cut circumferential grooves, 16 to the inch. Around this armature, and separated from it by only a narrow air gap, is an electromagnet coil structure with soft iron pole pieces with matching circumferential grooves, 16 to the inch. When a current is applied to this coil, the system becomes a variable reluctance motor which seeks the position of least reluctance and the armature and carriage are moved until the grooves are aligned. A second coil structure, identical to the first, is positioned 1/48 of an inch out of phase with the first, and the third identical coil structure is positioned 1/48 of an inch out of phase with the first but in the opposite direction from the second. By turning on the current to the second coil and turning off the current to the first, the armature will be induced to move 1/48 of an inch. By turning on current to the third coil and turning off the second, the armature will move an additional 1/48 of an inch and so forth. By properly sequencing the current to the three coils, this linear stepper system can be directed to any of the 48 positions per inch defined by the three coil structure.
One drawback of linear steppers of the type above described is that the armature material must be chosen for its magnetic properties rather than for the desired rates of thermal and hygroscopic expansion. Consequently, the pitch of the linear stepper varies with the materials expansion and contraction rates as the ambient temperature and humidity changes.
Another problem is that the electromagnetic coils must be positioned in close proximity to the armature, and the substantial heat generated by their operation expands and distorts both their supporting structures and the length and pitch of the armature. When one coil is turned on for any substantial length of time, for example to hold the transducers in a desired position, that portion of the armature adjacent the active coil is being heated while other portions are cooling. When the motor is sequenced to another position, another coil begins heating a different portion of the armature, and the motor changes its thermal and dimensional nature over a period of time.
In a motor such as this, the air gap must be wide enough to prevent even momentary physical contact between armature and stator. In addition, the carriage guidance system must have great lateral stiffness, for as the armature deviates from a central balanced magnetic position, magnetic attractions decline slightly on the wide gap side, but increase greatly on the narrow gap side. This increasing imbalance pulls the armature even further off center. To oppose these forces requires a stiff armature, very stiff guidance means, and a sufficient air gap.
To achieve reasonable efficiency, the size of the teeth in the magnetic structure must be proportionally bigger than the air gap. As the teeth become very small relative to the air gap, their interaction across the air gap becomes negligible. Consequently, linear stepper motors are not suitable for fine pitch systems such as 96 increments per inch of storage on the disc.
A further problem with linear stepper motors, is the large mass of their armatures. Moving this mass imparts substantial kinetic energy to the system, which must be removed, primarily by mechanical friction, before the system comes to rest. This requires a long settling time while the carriage oscillates about the desired position and comes to rest which causes relatively long delays before the system is ready for data transfer operations. Additionally, since the system is not balanced, the heavy armature becomes a substantial burden to be driven "uphill", if the carriage travel is not exactly horizontal.
To overcome some of the foregoing problems, rotary stepping motors in conjunction with some means to convert the rotary motion to linear motion have become widely used in the prior art as transducer positioners in low cost drives.
Because the magnetic structure is fairly compact, narrow air gaps can be achieved. When the structure expands due to heat generation, the expansion is equal in all directions, therefore the output which is angular in nature, is not distorted. Ball bearings provide a reasonably stiff support to resist unbalanced magnetic forces.
Rotary stepping motors have a number of limitations, among which are dead band and oscillation The static friction of the system defines a zone called the dead band, in which the motor has insufficient torque to initiate motion. Therefore, if the armature comes to a stop anywhere in the dead band, it will remain at that position. To minimize this positional indeterminancy, the motor's torque must be maximized, and the static friction minimized. If the friction load is very low, then the energy stored in the moving armature causes it to oscillate about its terminal position for unacceptably long periods. To reduce settling times to an acceptable level, a moderate amount of friction must be present, which in turn increases the dead band.
Another weakness in stepper motors used as precision positioners is pole shift. Two kinds of stepper motors are possible. The variable reluctance type utilizes magnetically soft iron structures in which magnetism is selectively induced by electromagnetic coils. The permanent magnet type utilizes magnetically hard material to permanently induce magnetism in magnetically soft iron structures, while additional magnetomotive forces are selectively generated by electromagnetic coils. Ideally the permanent magnets in such devices should be infinitely hard magnetically, and the magnetically soft structures should be infinitely soft magnetically. These ideals are not possible, and in low cost motors, the choice of material for magnetic structures is heavily influenced by cost, and magnetic hardness is not the primary concern. The magnetically soft structures, which should ideally be manufactured of ingot iron or electromagnet steel, are more often, for cost reasons, manufactured of low carbon steel. They are also generally stamped and formed without subsequent annealing. They are also frequently electroplated in baths that can add carbon to the steel. For all of these reasons, the supposedly magnetically soft structures in low cost motors are frequently moderately hard magnetically.
As a result, the magnitude and polarity of magnetism in portions of the magnetic structures and particularly, portions of the soft iron structures, are slightly and temporarily altered during operation. Consequently, if a rotary stepping motor, with holding current applied to one of its coils, is mechanically deflected a few degrees clockwise from that null point, and the mechanical torque is then slowly released, the motor will return to a null position. During this deflection, magnetomotive forces from the coil in combination with magnetomotive forces in the magnetically hard and soft structures, impresses a pattern of magnetic flux on the structure which is constantly changing until the system comes to a stop indicating that the internal forces are balanced. If the motor is then mechanically deflected a few degrees counter-clockwise from that pole position and the mechanical torque again slowly released it will return to a new null position far removed from the original null position. A small portion of this change in null position or "pole shift" was due to the width of the dead band, but the majority of this shift was due to the creation of a different pattern of magnetism, which resulted in a new point of balanced forces. The positional indeterminancy in low cost stepper motors due to pole shift, is typically on the order of 3% of a full step which can radically affect data storage.
Another problem with the use of low cost stepper motors as precision positioners, is the loose tolerances applied to their fabrication and assembly. Positional errors of 5% of a step or more from this cause are not uncommon.
In order to achieve a small dead band, high torque is required, which means the highest possible ampere turns in the electromagnetic coils. For a given volume available for the copper magnet wire, a trade off is made between ampere turns and the greatest heat build up that can be tolerated. Consequently, stepper motors are major heat sources, and unlike rotating motors that assist in cooling themselves by generating windage, the stationary stepper conducting a holding current through one of its windings becomes quite hot. To alleviate this heat problem, some drive circuits incorporate a time delay to reduce the normal drive current to a lower holding current after the motor has stopped for a specific time interval. This approach reduces the heat build up, but it increases the potential dead band width, and assumes that the armature has already come to rest within the original high current dead band, and that subsequent shock, vibrations, etc., will not deflect the armature into the new, wider, low current dead band. Although permanent magnet type steppers possess a low level of torque when no current is flowing through the coils, this has not been sufficient in prior art to permit turning off the holding current entirely.
A number of means have been devised in prior art for converting the rotary motion of the rotary stepper motor into linear motion. One of these is a spiral face cam mounted on the stepper motor shaft. A spherical follower attached to the carriage, is spring biased into the spiral "V" groove of the face cam. As the stepper motor rotates the spiral cam, the follower moves radially and the carriage thus moves lineally with it. When the follower is close to the axis of the spiral cam, the friction between the follower and the cam, as well as the reaction to the inertia and friction of the carriage, are all acting at a small radius, (moment arm) and therefore, the torque loads on the stepper motor are small. Consequently, damping is minimal and settling times are long. Conversely, when the follower reaches the outer portions of the spiral cam, the torque loads on the stepper are large, and the dead band is excessive.
Another problem with the spiral cam, is its close proximity to the stepper motor. Heat from the stepper motor is conducted through the motor shaft into the spiral cam. When the drive is first turned on, the stepper and cam are cold. Data written on the cold drive is located in the position dictated by the cold spiral cam. After the drive has been operating for a time, the spiral cam has been expanded by heat from the motor. The transducer directed to the same nominal data position will instead be displaced by the amount of thermal expansion.
Another problem with the spiral cam, is that it is positionally referenced from the stepper motor itself. Since the motor is a major heat source, the structure to which the motor is mounted becomes heated and dimensionally distorted by that heat. This distortion changes the basic reference point from which the carriage position is derived. As a result, spiral cams are not used in the more precise drives.
Another prior art design uses a miniature ball bearing cam follower which runs against a spiral face cam. A small extension spring biases the cam follower against one side of the face cam.
This design adds the inaccuracies of the cam follower to the other inaccuracies of the spiral cam approach. Also the extension spring acting against the slope of the cam applies a biasing torque against the motor. This torque works against the motor in one direction as the motor extends the spring and thus stores energy in the spring. In the other direction, the motor is augmented by the energy released by the relaxing spring. In an open loop stepper system such as this, such augmentation is particularily unwelcome, since the stepper must be operated at a much slower rate to avoid accidentally over accelerating and loosing proper synchronism. This is particularily a problem in this design due to the small amount of damping friction provided by the ball bearing cam follower which also causes substantial settling times.
Another prior art means of coupling a rotary stepper to a linear carriage has been a lead screw. In this mechanism, the stepper motor shaft is elongated several inches, and a helical groove or thread is cut into the surface of the shaft. A follower attached to the carriage is spring biased into this groove.
To achieve sufficient accuracy in this lead screw, the helical groove must be precision ground. To keep the polar inertia of the system low, the shaft diameter must be small. As a consequence, the depth of the ground groove is a substantial percentage of the shaft diameter, and the unbalanced stresses created by the asymetrical grinding on the periphery of the shaft distorts it, and maintaining lead screw straightness becomes a problem. Some lead screws are ground with a double pitch thread to create symetrical, balanced stresses to improve straightness.
The thread grinding process by which these lead screws are manufactured is a sequential process. The entire thread cannot be produced at the same time. Instead, the grinding wheel is first plunged into the material at one end of the thread, and then follows the helical path of the thread until reaching the opposite end of the thread, it is retracted. Heat is generated during the grinding process. Although an attempt is made to standardize the temperature of the blank shafts, the grinding wheel, and the cutting oil, nevertheless the shaft gains temperature during the grinding operation. Consequently, since the pitch of the ground thread is maintained as a constant during the grinding operation, after the part is completed and its temperature becomes equal along its length, the pitch at the beginning end of the ground thread is greater than the pitch at the completion end of the thread. This is because the pitch at the completion end was ground into a warmer expanded shaft which has since contracted and with it the pitch of the thread ground into its surface has contracted. Finishing the thread with a very light second grind could largely eliminate this problem, but is not economically practical in low cost applications. Even though the crudest form of thread grinding is used, these ground lead screws contribute substantially to the cost of the drives.
Since the lead screw is an extension of the motor shaft, the substantial heat of the stepper motor is conducted down the shaft, and radiated from the shaft. The result is a heat gradient down the length of the shaft. When the drive is first turned on, the pitch of the lead screw is as manufactured. After the drive heats up, the pitch is expanded greatly near the motor, and to a lesser degree at the end away from the motor. This loss of positional accuracy due to motor heat is independent of a similar loss of accuracy due to changes in ambient temperature. As the ambient temperature changes, the dimensions of the data disc, which is fabricated from biaxially oriented polyethylene terephthalate, change also. Ideally the changes in pitch of the lead screw from ambient temperature changes should match the changes in pitch of the data tracks from the same cause. This has not been accomplished in the prior art, and for practical manufacturing reasons, lead screws have been fabricated from stainless steel with thermal expansion rates different from that of the data discs.
In some designs the stepper motor becomes the point of reference from which the carriage is positioned by the lead screw. Since the stepper is a major heat source, this heat from the motor conducts into the structure to which the motor is attached, and through thermal expansion distorts that structure, thereby changing the reference point from which positioning begins and shifting the carriage position.
To eliminate this problem, and to minimize the effects of stepper motor heat on the lead screw accuracy, some designs float the motor on a flexible support, and reference the opposite end end of the lead screw. In these designs, the portion of the lead screw furthest away from the motor and therefore the coolest, is used for positioning the carriage, and since it is fairly cool, it creates only a small amount of distortion from thermal expansion in the structure to which it is attached. The drawbacks to this system are added complexity, and in some cases troublesome, once-around, elastic torque loads are generated. In those systems, three bearings are mounted on a slightly curved lead screw, and the motor which contains two of those bearings is supported on an elastically flexible mount. As the curved lead screw rotates, the motor is forced to wobble with it, and in the process alternately stores and releases energy in the elastically flexible mount. This alternating spring biasing of the torque output has the same negative effects as the extension spring in the face cam drive.
In this system, since the motor is flexibly mounted, rather than directly mounted to a solid supporting structure such as a cast chassis, the heat sink effect of such a chassis is lost, and since the flexible mount usually has less thermal conductivity, the stepper motor warms to a higher temperature, as does the lead screw. Thus, the thermal expansion of the lead screw pitch is even greater in this system.
When the biaxially oriented polyethylene terephthalate disc changes size in response to a change in ambient humidity, it is preferred, that the pitch of the lead screw would change by the same amount. Since the lead screws in prior art have been manufactured from stainless steel which is dimensionally unaffected by changes in humidity, no such hygroscopic compensation in the pitch of lead screws has been accomplished.
A further difficulty in lead screw designs is the proper tracking of the lead screw by the follower. In order for the follower to exactly duplicate the pitch of the lead screw, it must move exactly parallel with the axis of the lead screw. If the lead screw were exactly straight, this would present a substantial alignment problem, and since due to manufacturing limitations, the lead screws are frequently slightly curved, the problem is worse.
In the prior art, an attempt is usually made to minimize this alignment problem by mounting the carriage on the lead screw itself by means of a pair of bearings in addition to the follower which rides in the ground thread with these extra bearings riding frictionally against the outside diameter of the lead screw. As a result, the carriage moves laterally and vertically in response to rotation of the somewhat curved lead screw as a whole, while departures from proper pitch by the follower are limited to those caused by curvature of the lead screw only in the length between the two supporting bearings.
The addition of these extra carriage support bearings adds substantially to the frictional torque load which must be driven by the stepper motor, and that in turn increases the dead band.
Some of these lead screw designs utilize a 90 degree "V" thread profile, in which case when the carriage reaches the end of its travel, the follower is driven out of the thread without harm to the system. Other designs use 60 degree Acme thread profiles, and when the carriage in such a system is driven to the end of its travel, destructive forces are generated by the lead screw. Such systems are generally augmented by mechanical stops which impact a rotating element on the lead screw against some portion of the carriage to provide a less destructive stopping means. These stopping means still create high rates of decelleration, and large forces in the structure, and are a cause of failures and misalignments.
The third means of coupling the rotary stepper to the linear carriage in the prior art has been a flexible band. In this design, a cylindrical drum or capstan is mounted on the shaft of the stepper motor. A thin, flexible metal band encircles the capstan and is attached to the capstan at the midpoint of the band. By this means, when the stepper motor rotates, the flexible band moves with it carrying the carriage along as well.
The magnitude of movement of the carriage is a function of the pitch radius of the flexible band, which is the outside radius of the capstan plus one half of the band thickness. A problem with this design is that the capstan mounted directly onto the stepper motor shaft is heated by the motor and thermally expands, thus increasing the pitch.
Another problem is that the portion of the band that is wound around the capstan assumes nearly the same temperature as the capstan, while that portion of the band lying away from the capstan does not. As a result, when a hot capstan turns, it unwinds hot and therefore thermally expanded metal band, and winds in cool and therefore contracted metal band. After the capstan stops at the newly selected position, the hot newly unwound band begins to cool and contract. At the same time the cool, newly wound band draws heat from the capstan and begins to warm and expand. Tne result is that initial positioning by the stepper is exaggerated, followed by a slow correction as thermal equilibrium is again established.
Another weakness in this system is that the stepper motor is the basic reference point from which positioning is established, and the heat from the stepper motor is conducted into the supporting structure, thermally expanding and distorting it, thus changing the reference point and therefore the carriage position.
The capstan in such a system is a precision part, the diameter and concentricity of which must be very accurately established to closely coincide with the nominal pitch value. Consequently, existing designs have fabricated this part by precision grinding a metallic part. Ideally this part should expand thermally at the same rate as the biaxially oriented polyethylene terephthalate data disc to achieve exact compensation for changes in ambient temperature. The materials used for capstans in the prior art do not achieve this goal.
It is also desirable, that the capstan expand hygroscopically at the same rate as the disc, but this too has not been achieved in the prior art mechanisms. The metallic capstans are totally unaffected dimensionally by changes in humidity, and so the changes in data ring pitch are completely uncompensated.
Another problem with this design, is the short life of the flexible band. The maximum practical number of steps that can be provided in a stepper motor is approximately 200 per revolution. In order to couple such a 200 step motor to a 96 track per inch carriage, the capstan must have a pitch radius of 0.3316 inches. In order to flex the band around such a tight radius, even for a small number of cycles, the band must be very thin, stresses are high, and service life is limited. Because of these limitations, further pitch reductions are not practical with this system. A further limitation is the requirement, that the capstan must turn somewhat less than a full turn, otherwise the flexible band would start to wind on top of itself.
If the carriage in such a system is run to the end of its travel, and the rotating motor is stopped suddenly by the flexible band, it is overstressed and breaks the band. Consequently, stops must be added to the motor to limit its rotation. When the motor is run at full speed into one of these stops, rapid decelleration occurs. This sudden decelleration is conveyed to the carriage by means of the flexible band, and as the momentum of the carriage is suddenly applied to the band, high stresses are generated. The elements of this system are precise, fragile, and easily damaged during assembly.